1. Field of the Invention
This invention relates to a display device by cathodoluminescence excited by field emission, or cold emission, and more precisely a display screen comprising a microtips electron source observable through the microtips support and a process for manufacturing this source.
The invention is particularly applicable to the manufacture of matrix display devices for the display of fixed or animated images.
A screen conform with the invention comprises a partially transparent cathode structure.
Remember that the advantage of this type of partially transparent cathode structure is that it can be used to observe phosphors on the screen on the same side as the electrons that excite them, thus recovering more light and thus improving the light efficiency of the screen.
2. Discussion of the Background
The principle by which phosphors are observed from the side on which they are excited is known.
It is used particularly for VFD (Vacuum Fluorescent Display) devices.
The only difference between these devices and microtip screens is the method of emitting electrons.
FIG. 1 in the attached drawings diagrammatically shows the structure of a VFD.
In this VFD, an electrically insulating substrate P1 and a glass plate P2 delimit an area Z in which the vacuum is created and which is closed along its periphery by a sealed material M.
The area Z contains heating filaments F capable of emitting electrons by thermionic effect.
Cathode conductors C made of aluminum are formed on the substrate P1 facing these heating filaments F, and are covered with phosphors P.
Light L emitted by these phosphors is observed at 0 through the glass plate P2.
Furthermore, a grid G placed between the heating filaments F and the cathode conductors C modulates the electronic current.
The principle mentioned above has also been used in the recent past for plasma color display screens.
Document (1) French patent application deposited on Jul. 27, 1984 No. 8411986 also describes a microtips screen structure in which the phosphors are observed on the side on which they are excited.
Document (2) international application PCT/US91/04491 by COLORAY DISPLAY CORPORATION, international publication number WO92/00600, describes the observation of phosphors from the side on which they are excited through a "transparent" cathode in a microtips screen.
The structure described in this document (2) is composed of metallic rows and columns which are placed at a sufficient spacing so that the cathode can transmit 80% of the light.
Under these conditions, the area covered by the microtips only occupies 1% of the area of the cathode, which considerably reduces the average effect and makes higher addressing voltages necessary to obtain the necessary electronic current.
Furthermore, this cathode does not have a meshed structure or a resistive layer.
A display screen with a partially transparent cathode provided with a resistive and meshed structure is described in document (3) French patent application No. 9202220 Feb. 26, 1992 (FR-A-2687839) corresponding to EP-A-0558393 and the American patent application Ser. No. 08/022,935 (Leroux et al.), Feb. 26, 1993.
This document (3) is included in this description by reference.
The partially transparent cathode described in this document (3) is based on a perforated grid structure associated with a transparent resistive layer.
This is shown in FIGS. 5 and 6 in document (3).
This type of structure requires the development of a resistive material that must have a suitable resistivity (of the order of 10.sup.3 to 10.sup.4 .OMEGA..cm) and a high transmission in the visible range (greater than 80%).
This material is difficult to make and particularly difficult to reproduce in a controlled and uniform manner on large areas.